The rate at which some weathering steels form the desired patina is greatly influenced by the presence of atmospheric pollutants which catalyze corrosion. While the process is generally successful in large urban centers, the weathering rate is much slower in more rural environments.
As indicated earlier, weathering steel forms an adherent protective rust ‘patina’ in suitable environments that inhibits further corrosion. According to SteelConstruction.info, the corrosion rate is so low that bridges fabricated from unpainted weathering steel can achieve a 120-year design life with only nominal maintenance. Hence, a well-detailed weathering steel bridge in an appropriate environment provides a very low maintenance, economic solution. In the presence of moisture and air, all low alloy steels rust, the rate of which depends on the access of oxygen, moisture and atmospheric contaminants to the metal surface. As the process progresses, the rust layer forms a barrier to the ingress of oxygen, moisture and contaminants, and the rate of rusting slows down.